DESCRIPTION

Condition variables are used in conjunction with mutexes to wait for
conditions to occur. Condition variables are created with cv_init(),
where cvp is a pointer to space for a structcv, and desc is a pointer to
a null-terminated character string that describes the condition variable.
Condition variables are destroyed with cv_destroy(). Threads wait on
condition variables by calling cv_wait(), cv_wait_sig(),
cv_wait_unlock(), cv_timedwait(), or cv_timedwait_sig(). Threads unblock
waiters by calling cv_signal() to unblock one waiter, or cv_broadcast()
or cv_broadcastpri() to unblock all waiters. In addition to waking
waiters, cv_broadcastpri() ensures that all of the waiters have a
priority of at least pri by raising the priority of any threads that do
not. cv_wmesg() returns the description string of cvp, as set by the
initial call to cv_init().
The lock argument is a pointer to either a mutex(9), rwlock(9), or sx(9)
lock. A mutex(9) argument must be initialized with MTX_DEF and not
MTX_SPIN. A thread must hold lock before calling cv_wait(),
cv_wait_sig(), cv_wait_unlock(), cv_timedwait(), or cv_timedwait_sig().
When a thread waits on a condition, lock is atomically released before
the thread is blocked, then reacquired before the function call returns.
In addition, the thread will fully drop the Giant mutex (even if
recursed) while the it is suspended and will reacquire the Giant mutex
before the function returns. The cv_wait_unlock() function does not
reacquire the lock before returning. Note that the Giant mutex may be
specified as lock. However, Giant may not be used as lock for the
cv_wait_unlock() function. All waiters must pass the same lock in
conjunction with cvp.
When cv_wait(), cv_wait_sig(), cv_wait_unlock(), cv_timedwait(), and
cv_timedwait_sig() unblock, their calling threads are made runnable.
cv_timedwait() and cv_timedwait_sig() wait for at most timo / HZ seconds
before being unblocked and returning EWOULDBLOCK; otherwise, they return
0. cv_wait_sig() and cv_timedwait_sig() return prematurely with a value
of EINTR or ERESTART if a signal is caught, or 0 if signaled via
cv_signal() or cv_broadcast().

RETURNVALUES

If successful, cv_wait_sig(), cv_timedwait(), and cv_timedwait_sig()
return 0. Otherwise, a non-zero error code is returned.
cv_wmesg() returns the description string that was passed to cv_init().

ERRORS

cv_wait_sig() and cv_timedwait_sig() will fail if:
[EINTR] A signal was caught and the system call should be
interrupted.
[ERESTART] A signal was caught and the system call should be
restarted.
cv_timedwait() and cv_timedwait_sig() will fail if:
[EWOULDBLOCK] Timeout expired.